1. Field of the Invention
This invention relates to an improved stereo indicating circuit for use with an FM receiver having a multiplex demodulator, a stereo indicator and a stereo-monaural mode selector.
2. Discussion of the Prior Art
In most FM stereo receivers, a stereo indicator such as a beacon lamp BL and a stereo-monaural mode selector SM are connected to a multiplex demodulator MPX as shown in FIG. 1 where the receiver is in its monaural mode when switch SM is closed and in its stereo mode when SM is open. Beacon lamp BL is also energized when the receiver is in its stereo mode. A smoothing circuit (not shown) applies the +B voltage to light the stereo indicator BL. In some prior art receivers, the time constant of the smoothing circuit is relatively short to prevent the stereo indicator from continuing to be lit for a while after the power switch of the FM receiver is turned off. However, there are certain problems in using a smoothing circuit having a short time constant to derive the +B voltage which lights the stereo indicator BL. Thus, the output voltage from a smoothing circuit with short time constant has ripples which undesirably affect the multiplex demodulator and cause some problems in stereo separation and distortion of the receiver. However, if the time constant of the smoothing circuit is too long, as in the case in some prior art receivers, BL tends to stay on after the main power switch has been turned off, as mentioned above.